The present invention is directed to an apparatus for laying cables in conduits such as for laying electrical cables in underground ducts.
Electric utilities lay a large quantity of cables in underground ducts. Generally three cables are laid simultaneously for three-phase electrical power. A typical duct has an internal diameter of about 5 inches, and the cable diameter typically is about 13/4 inches for 1,000,000 circular mil cable. In order to lay three cables simultaneously, cable manufacturers provide three lengths of cables helically twisted about each other into what is commonly referred to as a "triplex" cable.
To lay the triplex cable, it is pulled through the conduit by means of a rope. At the feed end of the conduit, a split bell is provided for guiding the triplex. The bell is conventionally provided with a grease fitting for radial introduction of lubricant to aid in pulling the triplex cable through the conduit.
There are many problems with this type of system. For example, it is difficult to pull the triplex cable through a conduit because the lubricant is applied only on the exterior surface of the cables. The high pulling forces required can stretch the cables, resulting in broken strands and line discontinuities. Another disadvantage of triplexing is that it is expensive, costing about 15 to 20 cents per linear foot.
A further disadvantage of the triplex cable system is that there can be substantial wastage of cable. If 600 feet of triplex are ordered, but only 500 feet are actually required, there are 100 feet of the triplex cable left over. This leftover triplex cable has little, if any use, because it cannot be spliced to other triplex cable because a large splice cannot be pulled through a conventional underground duct.
Another problem with triplex cable is that most manufacturers only supply it in lengths up to about 900 feet, because that is the maximum that can be placed on a single reel. Therefore, for lengths longer than 900 feet, it is necessary to build splicing vaults into the underground duct system. The splicing vaults are expensive to construct, the labor for forming the splices is expensive, and the splices are often a source of line discontinuities.
In view of the problems associated with pulling triplex cable through a duct, it is evident that there is a need for a system that allows three cables to be laid in a duct easily, inexpensively, and with little waste cable.